Kat's advice was right on!  The way you react to this situation could be a crossroads for your relationship with your son.  As a teenager, he is less likely to respond favorably to you getting upset with him for having sex.  The best way to handle it is to approach him in a loving and informational way, showing concern about the potential consequence rather than "judging" him or his behavior.  
 
Just as Kat mentioned, your response will depend on your personal, family, and religious beliefs.  However, whatever position you choose to take, make sure you talk to him in more of a conversational way rather than condeming him for the choices he has made.  What's done is done.  Neither you nor he can go back and change the fact that he had sex.  Now is the time to educate him about what it means to make mature, adult decisions.  Give him lots of information on STD's and pregnancy.  If he acts embarassed or isn't willing to listen to you talk about such sensitive subjects, remind him that if he's not mature enough to have an adult conversation about it, that's the first sign that he shouldn't be doing it.  Also talk to him about what it means to be in a committed relationship and the emotions and consequences that come along with being in a sexual relationship.  
 
Ultimately the choice is up to him.  The more you arm him with information the more likely he is to make solid decisions in the future.
 
Good Luck!
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